In a plant for power transmission by means of high-voltage direct current, switchings of the direct current in a converter station between different current paths are made in certain operating cases.
In a monopolar HVDC transmission, or during monopolar operation of a bipolar transmission, the operating current of a converter station flows in undisturbed operation via a ground electrode. Normally, this ground electrode is located at a distance from the station and is connected thereto through an electrode line, often of considerable length. An HVDC station normally has a neutral busbar, by which is meant in this application a busbar in an HVDC station which is intended, during normal operation, to lie at a potential near ground. Such a busbar is connected to or connectible to the ground electrode via the electrode line. Neutral busbars normally exist in the converter stations both in monopolar and in bipolar HVDC transmissions.
In connection with certain faults, for example an interruption in the electrode line, it is known to temporarily connect the neutral busbar to a station ground with the aid of a mechanical circuit breaker, the station ground being a ground electrode arranged in the station or near the station. After termination of the disturbance, the circuit breaker has been opened, whereby the operating d.c. current has been transferred from the station ground back to the electrode line and the ordinary ground electrode. This has caused high overvoltages to arise on the neutral busbar because of the impedance of the electrode line, which has necessitated overvoltage protective devices in the form of surge arresters, which have had to be amply dimensioned both from the point of view of voltage and energy. The overvoltage protective devices have therefore become costly. Further, the mechanical circuit breakers have constituted an undesirable complication of an HVDC station.
Because of the impedance of the electrode line, overvoltages on the neutral busbar in an HVDC station can also occur at an overcurrent on the electrode line. Also such overvoltages necessitate amply dimensioned surge arresters in known HVDC stations.
Similar problems arise if, in case of a fault in one of the poles of a bipolar transmission, it is desired to transfer the direct current from this pole to the electrode line and the ground electrode to be able to continue the operation of the plant as monopolar operation with the faultless pole. The case will be the same if it is desired to transfer the direct current back from the electrode line to a so-called metallic return, that is, to the line of the faulty pole, for a more permanent monopolar operation. Similar problems also arise if, in a bipolar transmission, during monopolar operation with a metallic return, it is desired to transfer the operating current to the electrode line, for example as a first step in a change to a bipolar operation.
The problems have been described above with particular reference to a plant for power transmission by means of high-voltage direct current, but similar problems arise generally in d.c. circuits when transferring a direct current from one current path to another, whereby in a known manner the circuit impedances, and in particular the circuit inductances, render the transfer difficult and cause overvoltages.